The present invention relates generally to the manufacture of integrated circuits and more particularly to integrated circuits employing floating gates and control gates that are capacitively coupled to one another.
Currently, many non-volatile memory devices have information storage cells that employ xe2x80x9cfloatingxe2x80x9d gate structures. Floating gates are created by electrically isolating an electron storing structure by one or more dielectric layers. Charge can then be placed on, or removed from the floating gate. The resulting charge on the floating gate is used to alter the memory cell parameters. For example, a single transistor storage cell is created by situating the floating gate in-between a control gate and its corresponding transistor channel. According to the charge on the floating gate, the threshold of the transistor is altered. For many non-volatile memories a cell is xe2x80x9cprogrammedxe2x80x9d by placing electrons on a floating gate, and xe2x80x9cerasedxe2x80x9d by removing electrons from the gate.
A discussion of electron transport mechanisms used in the programming and erasure of non-volatile memories is set forth in U.S. Pat. No. 4,328,565, issued to Eliyahou Harari, on May 4, 1982. Harari notes that for erasable programmable read only memories (EPROMs), channel hot electron injection is used to place electrons on floating gates, while the application of ultraviolet light, or Fowler-Nordheim tunneling, can be used to remove electrons from the floating gate. Hot electron injection usually involves raising the control gate and drain to a relatively high potential with respect to the source. Fowler-Nordheim tunneling is accomplished by applying a strong electric field between the floating gate and the control gate or substrate. For example, a negative gate voltage between the source and the control gate. Harari notes that an important factor in programming EPROM cells is the capacitive coupling between the control gate and the floating gate. In addition, Harari indicates that the capacitive coupling is dependent upon the geometric overlap between the floating gate and the control gate, and the nature of the dielectric therebetween. (This dielectric is often referred to as the xe2x80x9cinter-polyxe2x80x9d dielectric in those structures that employ polysilicon floating gates and control gates. For the purposes of this description the dielectric separating the control gate from the floating gate will be referred to in this description as an xe2x80x9cintergatexe2x80x9d dielectric.)
U.S. Pat. No. 4,713,677 issued to Tigelaar et al. on Dec. 15, 1987 notes that the. tunneling voltage increases as the capacitive coupling between the control gate and floating gate increases. Tigelaar et al. discloses an electricaly erasable PROM (EEPROM) wherein the capacitive coupling between the floating gate and control gate is increased by a trench capacitor adjacent to the active area on the floating gate.
Other approaches for increasing the capacitive coupling between the control gate and the floating gate are known in the prior art. U.S. Pat. No. 4,169,291 issued to Bemward Rossler on Sep. 25, 1979 illustrates a xe2x80x9cV-MOSxe2x80x9d EPROM cell. The geometric overlap between the floating gate and the control gate is increased by forming the transistor within a groove etched into the substrate.
U.S. Pat. No. 5,143,860 issued to Mitchell et al. on Sep. 1, 1992 discloses an EPROM cell having sidewall floating gates. The sidewall floating gate arrangement increases the surface area of the floating gate that is adjacent to the control gate, and thus increases the capacitive coupling between the two.
U.S. Pat. No. 5,343,063 issued to Yuan et al. on Aug. 30, 1994 discloses a read only memory cell in which a floating gate is conformally formed within a thick dielectric trench. The control gate is then formed over the floating gate. The overlap area is increased due to the vertical extent of the thick dielectric trenches.
U.S. Pat. No. 5,378,643 issued to Ajika et al. on Jan. 3, 1995 discloses a flash EEPROM (EPROM) in which the control gate is formed by two layers of polysilicon, with the two layers surrounding the floating gate. By surrounding the floating gate with the control gate, effective overlap area between the control gate and the floating gate is increased.
U.S. Pat. No. 5,382,540 issued to Sharma et al. on Jan. 17, 1995 discloses a flash EEPROM (EPROM) in which the source, channel and drain are formed in epitaxial silicon pillars. The floating gates are formed by polysilicon sidewalls that surround the pillars. The control gate is formed over the entire pillar structure. The unique Sharna et al. device also increases capacitive coupling between the control gate and the floating gate by maximizing the surface area between the two. The conventional one transistor (1-T) flash EPROM cell is set forth in U.S. Pat. No. 4,698,787 issued to Mukherjee et al. on Oct. 6, 1987.
Commonly owned, co-pending U.S. patent application Ser. No. 08/456,080 entitled DRAM CELL WITH SELF-ALIGNED CONTACT AND METHOD OF FABRICATING SAME discloses a dynamic random access memory (DRAM) cell in which the DRAM capacitor area is increased by an initial, xe2x80x9cthickxe2x80x9d conductive layer.
While the prior art provides a number of approaches to increasing the surface area between a floating gate and its associated control gate, such approaches require complicated process technologies that substantially deviate from the current 1-T cell manufacturing processes. Further, increasing the overlap area between floating gates and control gates by trenches and similar techniques can result in poor intergate dielectric integrity, limiting the endurance and reliability of the device.
It is an object of the present invention to provide a non-volatile memory cell having increased capacitive coupling between a control gate an its associated floating gate(s).
It is another object of the present invention to provide a non-volatile memory cell having increased capacitive coupling between a control gate an its associated floating gate(s) that can be easily substituted for conventional, non-volatile memory cells.
It is another object of the present invention to provide a method of manufacturing a non-volatile memory cell having increased capacitive coupling between a control gate an its associated floating gate(s) that is easily integrated into conventional non-volatile memory fabrication processes.
The present invention includes an EPROM, EEPROM or flash EPROM cell having a vertically extending structure adjacent to a transistor channel. A floating gate is formed that overlaps both the channel and the vertically extending structure. A control gate is formed over the floating gate. The addition of the vertically extending structure increases the overlap area between the control gate and floating gate, which, in turn, increases the capacitive coupling between the two.
According to one aspect of the present invention, the vertically extending structure is formed from an initial, relatively thick layer of polysilicon.
According to another aspect of the present invention, the vertically extending structure is patterned from a deposited layer using the floating gate etch mask.
According to another aspect of the present invention, a 1-T flash EPROM includes a channel separated by field oxide regions in the channel width direction. Relatively thick polysilicon structures are formed on the opposing field oxide regions, and a floating gate conformally overlaps the thick polysilicon structures and the channel. A control gate conformally covers the floating gate.
According to another aspect of the present invention, the vertically extending structures are formed from a dielectric layer.
According to another aspect of the present invention, the floating gate is a multi-layered structure. A first floating gate layer is formed over a tunnel dielectric. The vertically extending structure is formed on the first floating gate layer, and a second floating gate layer overlaps the vertically extending structure and exposed portions of the first floating gate layer.
An advantage of the present invention is that it provides a one transistor flash EPROM cell wherein the polysilicon coupling between the control gate and the floating gate can be increased without increasing the size of the EPROM cell along the channel width direction.
Another advantage of the present invention is that it provides a one transistor flash EPROM cell that can be reduced in size along the transistor channel width direction.
Yet another advantage of the present invention is that it provides a method of fabricating a one transistor flash EPROM cell that does not require complicated trench etching steps.
Yet another advantage of the present invention is that it provides a one transistor flash EPROM cell that can be fabricated using conventional one transistor flash EPROM fabrication process.
Other objects and advantages of the present invention will become apparent in light of the following description thereof.